Corrugated pipe has been used in the drainage of water-saturated soil in various agricultural, residential, recreational, or civil engineering and construction applications, such as for storm sewers. Traditionally, drainage pipe was made from clay or concrete, which caused the pipe to be heavy, expensive, and brittle. In order to improve the cost-effectiveness, durability, and ease-of-installation of drainage pipes, it is now common in the art to manufacture them from various materials including various polymers and polymer blends. Such polymer pipes are typically corrugated, having a molded profile with sides of the corrugation that are fairly steep and a top, or crest, of the corrugation that is fairly flat.
There are two basic ways that polymer, corrugated pipe can fail in use: by deforming excessively or by fracturing. Stiffer material is less likely to deform but more likely to fracture under stress. Flexible material is more likely to deform but less likely to fracture under stress. Deformation is expressed as a ratio of elongation of the material to its original material length and is called “strain.” Stress causes the deformation that produces strain. The modulus, or stiffness, of a plastic is the ratio of stress divided by strain, or the amount of stress required to produce a given strain.
There are a number of ways to provide lower deformation of a pipe in use: (1) increasing pipe stiffness by using a stiffer material; (2) thickening the pipe walls; or (3) changing the wall design to increase the moment of inertia, which increases the overall stiffness of the pipe wall. Using stiffer material to make a corrugated plastic pipe is disadvantageous because the pipe must be able to deflect under load to a certain degree without cracking or buckling. A certain amount of elasticity is therefore beneficial in preventing brittle failures upon deflection.
Thickening the pipe walls is also disadvantageous because it adds material cost and increases weight to the pipe, which increases shipping and handling costs. Thus, it is advantageous to find a wall design that increases the moment of inertia of the pipe, while causing a minimal increase to the weight of the pipe or the stiffness of the material used to make the pipe.
Increasing the moment of inertia of a pipe wall increases its resistance to bending. One example of a wall design that increases the moment of inertia, and therefore the stiffness, of a plastic corrugated pipe with minimal increase in pipe weight and material stiffness is illustrated in U.S. Pat. No. 6,644,357 to Goddard. In this pipe, the ratio of the height of a corrugation to the width of that corrugation is less than 0.8:1.0, and the sidewall of the corrugation is inclined, with respect to the pipe's inner wall, in the range of 75-80°. This ratio allows the pipe to deflect to greater than 30% of its original diameter without exhibiting imperfections associated with structural failure.
Pipe failure can be prevented by minimizing the maximum force exerted on the pipe walls during the bending associated with deformation. If a sheet of material, such as plastic, is flexed, the outside of the resulting curve is deformed in tension, and the inside of the curve is deformed in compression. Somewhere near the middle of a solid sheet is a neutral plane called the centroid of the sheet. In the case of corrugated pipe, the “sheet” thickness comprises corrugations to achieve economy of material. Because the “sheet” is therefore not solid, the centroid may not be in the middle of the sheet, but rather is located at the center of the radius of gyration of the mass (i.e., the centroid is displaced toward the location of greater mass). The more offset the centroid is from the middle of the sheet thickness, the greater the maximum force will be at the surface farthest from the centroid during bending or flexure from deformation, due to a longer moment arm for certain acting forces. Thus, to lower the maximum force caused by pipe wall deformation, the pipe should be designed so that the centroid is closer to the middle of the sheet thickness. The closer the centroid is to the middle of the sheet thickness, the more desirably uniform the stress distribution will be. Thus, the maximum stress upon deformation will be minimized to prevent pipe failure due to shorter moment arms for acting forces.
FIG. 1 illustrates a vertical cross-section of a sidewall section of one type of prior art double-wall corrugated pipe. The illustrated section includes a smooth inner wall 100 and a corrugated outer wall 110. The corrugated outer wall includes corrugation crests 120 and corrugation valleys 130.
In use, it is the deflection and integrity of inner wall 100 that is critical to pipe performance. Deflection of the outer wall 110 is greater than deflection of the inner wall 100 in use, but a certain amount of deflection of the corrugated outer wall 110 is acceptable because, although maintaining the integrity of the outer wall 110 is advantageous, its integrity can be sacrificed to a certain extent without affecting pipe performance, as long as the integrity of the inner wall 100 is maintained. Thus, it is advantageous to provide some flexibility in the outer wall 110 so that it can deflect in use without that deflection translating to the inner wall 100. Although the double wall pipe illustrated in FIG. 1 may have sufficient flexibility, its centroid is too far from the middle of its sheet thickness to provide sufficiently uniform stress distribution during deformation. Moreover, the double wall pipe profile provides insufficient resistance to pipe buckling, for a given amount of raw material. Therefore, the double wall pipe may not be stiff enough to provide installation insensitivity and long-term durability.
Accordingly, it would be advantageous to provide a corrugated polymer pipe having an additional outer layer that increases the moment of inertia so the pipe experiences less deformation in use, and greater resistance to buckling.